The present invention relates to electrical generators, and more particularly to methods and systems for monitoring the condition of bearing insulation in an electrical generator.
Rotors in large synchronous electrical generators are supported at each end by generator bearings. Generator bearings provide a smooth-lubricated surface for rotors to spin on. Generator bearings typically must be insulated in order to electrically isolate the rotor from ground and from the rest of the generator. This electrical isolation prevents naturally occurring voltages on the rotor from shorting through the bearings. Such shorts could damage the surface of the bearings and lead to bearing wipe, which would significantly damage the generator.
Shorts in bearing insulation may be caused by a number of different situations. Shorts may be caused, for example, by damage to the bearing insulation itself, by improperly insulated bearing instrumentation (such as bearing thermocouples), or by a bridge across the bearing insulation created by oil residue, dirt, paint, or a repair tool left in the generator. One conventional approach for identifying bearing insulation problems in a generator that is operating is to test of the value of the resistance of the insulation with a device called a Meggar(trademark). However, this test is only possible on certain bearing configurations. It will not work, for example, with a bearing configuration known as a single-insulation bearing.
Various attempts have been made to detect problems in a generator while the generator is operating. U.S. Pat. No. 5,032,826 to Twerdochlib, for example, which is incorporated by reference herein in its entirety, discloses a monitor for detecting generator core hot spots by monitoring rotor shaft voltage. However, a need remains for a reliable technique for monitoring bearing insulation while a generator is in operation in a way that is not restricted to specific bearing configurations.
With the foregoing in mind, methods and systems consistent with the present invention provide a means of predicting problems with bearing insulation in an electrical generator, such as insulation damage or bridging, to provide an indication of the problem to an operator and thereby prevent possible damage to an electrical generator. This and other objects, features, and advantages in accordance with the present invention are provided by a method that comprises the steps of monitoring the shaft voltage occurring over the length of the rotor of the electrical generator; and setting, when the shaft voltage drops below a predetermined value, an alarm to indicate to an operator that a problem exists.
A monitoring system consistent with the present invention is provided for identifying a problem with bearing insulation in an electrical generator. The monitoring system comprises means for monitoring shaft voltage occurring over the length of the electrical generator on a rotor of the electrical generator; and means for setting, when the shaft voltage drops below a predetermined value, an alarm to indicate to an operator that a problem exists.
A monitoring system consistent with the present invention is also provided for identifying a problem with bearing insulation in an electrical generator that comprises at least one shaft-riding brush mounted to the rotor of the electrical generator for providing an indication of a shaft voltage across the electrical generator; an analog-to-digital converter for converting the shaft voltage to a digital signal; and a data-processing system for monitoring the shaft voltage and for setting an alarm when the shaft voltage drops below about 5 volts.